This invention complements the invention described and claimed in applicant's U.S. Pat. No. 4,858,543 issued Aug. 22, 1989 and assigned to the assignee of the present invention. The bobbin basket covered by that patent is adapted for use with a horizontal rotary loop taker, which is the term used in the sewing machine industry for rotary loop takers or "hooks" that when installed in a lock-stitch sewing machine have a horizontal axis of rotation. The present invention is useful for any bobbin basket, whether it is adapted for use with a horizontal hook or with a vertical hook.
Prior to the invention disclosed and claimed in U.S. Pat. No. 4,858,543, bobbin baskets used in the industrial sewing machine industry were customarily made of metal, typically steel. Steel bobbin baskets have a number of disadvantages that have been recognized for a very long time. The invention covered by U.S. Pat. No. 4,858,543 took a big step toward providing a bobbin basket that avoids the problems associated with steel bobbin baskets, by providing a plastic bobbin basket of greatly increased usefulness and much longer life than was previously believed to be possible. The present invention goes still farther in increasing the usefulness of a plastic bobbin basket and extending its life.
The many advantages that a bobbin basket formed almost entirely of plastic has over a conventional all steel basket are clear. Molding of the basic plastic piece (with or without the small inserts of the present invention which are described below) is far more convenient and less expensive than the casting or forging--followed by various machining, polishing and hardening steps--that is involved in the production of an all steel bobbin basket. Chattering of the bearing rib in its raceway--which produces chipping and burring, with resulting thread breakage--is avoided with such a plastic bobbin basket. The lower coefficient of friction between the bearing rib and its raceway produces a longer life for the plastic bobbin basket, and avoids the problems of messy lubrication, galling, bluing of the rotary loop taker, and puckering of the goods being sewn that accompany the use of an all steel basket.
As already mentioned above, U.S. Pat. No. 4,858,543 took a big step towards making all these advantages available to the sewing industry. The present invention provides another highly significant advance toward eliminating various types of wear and impact damage that have over the years seemed to deter those skilled in the art from attempting to achieve the highly desirable but presumably impossible goal of a bobbin basket formed almost entirely of plastic.
A number of prior workers in this field have fabricated bobbin baskets partly of plastic and partly of metal, but none of them have utilized this approach to produce a bobbin basket that is formed almost entirely of plastic, as applicant does. Examples of the universal failure to realize that the many significant advantages of a bobbin basket formed almost entirely of plastic are capable of achievement are provided by U.S. Pat. No. 4,108,095 to Kohara, U.S. Pat. No. 4,377,121 to Teranishi, U.S. Pat. No. 4,393,798 to Cheng and U.S. Pat. No. 4,700,643 to Hirose et al. In Kohara, bobbin carrier 32 is comprised of upper member 33, lower member 34 and intermediate member 35, with only the latter member being formed of molded plastic material (col. 2, lines 40-46). In Teranishi, bobbin case 12 is divided into an upper portion 13 made of metal, and a lower portion 14 made of plastic (col. 1, lines 58-60). Cheng's bobbin basket is made up of an upper part, a lower part, and a middle part that constitutes the bearing rib, with that rib being either formed of, or coated with, Teflon (col. 2, line 13). In Hirose et al., only track projection member 27 (the equivalent of a bobbin basket bearing rib) is made of synthetic resin material (col. 4, lines 22-24). It is evident that none of these prior workers in this field recognized the extreme value, or even the possibility, of fabricating a bobbin basket almost entirely of plastic.
U.S. Pat. No. 4,676,178 to Hirose deals with the problem of frictional wear between the bobbin basket bearing rib and its associated raceway by an entirely different, complicated and more expensive expedient--the use of magnets embedded in the bearing rib and raceway with identical magnetic poles opposite each other to produce a repelling force that is expected to keep the rib and raceway from contacting each other. Whether or not this approach is useful in practical application, it is entirely different from applicant's invention.